Blade damping is an important feature that increases reliability in turbomachines by limiting the level of vibration created in rotating blades. Currently, blade damping in turbomachines is generally accomplished by inducing rubbing motion between contacting parts, causing frictional losses in the kinetic energy of the blades and thus reducing their level of vibration. The portions of the system that are caused to rub for damping purposes are typically placed in four locations. The first portion is a dove tail attachment used to fasten blades to disks or casings in turbomachines by means of interlocking serrations of contacting material at the blade base or root. The friction produced when the blade vibrates acts to dissipate kinetic energy and thus reduce vibration level. The second portion is a platform damper which uses the blade platform as a friction surface to dissipate kinetic energy and reduce vibration level by rubbing against the platforms of adjacent blades. The third portion is a mid-span shroud segment attached to each blade at the mid-way location of the blade span, as opposed to the base, which acts to bridge adjacent blades and causes contacting surfaces to rub and dissipate kinetic energy. Finally tip shrouds, having rubbing segments located at the blade tips that abut adjacent blade tip shrouds, produce a rubbing action that dissipates energy and reduces vibration.
Frictional dampers, such as those referred to above, derive their energy dissipation qualities from relative motion between contacting surfaces. Such mechanisms are not perfectly reliable and depend upon the maintenance of stable contacting force levels for their proper functioning. Since rubbing parts are prone to wear away over time, the contact force stability will decay with use causing a loss in damping effectivity. This limits damping to undesirably low levels, too low to prevent a significant shortening of blade life. The alternative is to use more frequent maintenance and live with reduced damping for some limited time. These compromises result in sub-par damper effectiveness, reduced machine safety and increased cost of maintenance.
There are other problems associated with frictional dampers now use(]on blades of turbomachines. Frictional dampers reduce blade performance. The blades are intended to effect an interchange of work between a flowing medium and the rotating shaft output. The use of mid-span or blade tip shrouds will always degrade that function to some degree by creating flow disturbances which causes some performance loss to occur. Additionally, such frictional dampers create manufacturing problems. A currently desirable means of manufacture, related to integral blade/disk assemblies, is not compatible with the use of frictional damping. The use of an integrally cast structure, where the blades and disk are created as one part, offers desirable cost advantages but does not facilitate the creation of adjacent rubbing surfaces of the type discussed above.
Accordingly, it is desirable to have a system of damping that allows integral cast structures, reduces frequent repairs due to wear and allows for increased engine efficiency by removing shrouds which reduce blade performance.
U.S. Pat. No. 4,722,668 discloses a device for damping blade vibrations in turbo-machine by equipping shroud plates on the blades with permanent magnet inserts. This dampens the blade vibrations due to the magnetic force between the individually paired magnets which keeps the shroud plates together or separated depending upon the polarity of the two magnets.
However, the '668 patent still requires shrouds which reduce blade performance. The extra mass of the magnets on the blades also increases the level of stresses on the blades reducing performance and lowering fatigue margins. Permanent magnets located on a turbine blade would also be severely degraded by high temperatures within a gas turbine.